The present invention relates to a gas turbine and, more particularly, to a gas turbine provided with a means for controlling a quantity of compressed air supplied from a compressor to a combustor of the gas turbine.
In recent years, as fuel supplied to a combustor of a gas turbine and burned, various fuels such as a gas generated in a coal gasification furnace, a gas (byproduct gas) generated in a blast furnace or a coke oven, etc. are used. However, such gases each have a property that a calorific value is lower than a usual fuel. JP A 60-222531 discloses that a fuel air ratio is controlled to be proper by providing an air compressor and fuel compressor provided with multi-stage variable vanes and controlling the multi-stage variable vanes according to a calorific value of a fuel to be burnt in the combustor.
However, in the above-mentioned apparatus, a construction of the compressor itself is been made complicated. Further, a usual fuel such as LNG has a calorific value of about 10,000 kcal/kg, while a calorific value of a byproduct gas from a blast furnace, etc. is about 1,000 kcal/kg which is low and about 1/10 of that of the usual fuel. For generating a high temperature high pressure gas by combustion of a fuel gas of low calorific value in the same manner as when a usual fuel is supplied, it is necessary to inject the fuel gas at a larger flow rate (about 10 times in this case) than the usual fuel flow rate according to the calorific value of the fuel gas. When a large amount of combustion gas flows into the turbine in this manner, a discharge pressure of the compressor rises and a margin for surging of the compressor decreases. On the contrary, when a compressor of smaller air capacity than a gas turbine compressor used for a usual fuel is used to make a discharge pressure constant so that a quantity of combustion gas flowed into the turbine becomes the same as that in the turbine used for a usual fuel, such requirement can not be satisfied by only one such compressor in a case where a plurality of fuel gases different in calorific value are supplied to the combustor.